A lot of the times the shelf life is a minimum of 1 year from the published date of manufacture in normal indoor storage conditions, but it could be more - sometimes up to 5 years. Typically, the bags are reusable.
For full storage/shelf life parameters for the Statshield series from Desco, see the following link:
Statshield EMI/RFI Moisture Barrier Bag Series Datasheet
To see Digi-Key’s full selection of moisture barrier bags from Desco, see below:
Desco Moisture Barrier Bags